1. Field of the Invention
The present invention relates to a wafer in which a plurality of nitride semiconductor laser devices to be separated from one another are arranged, and to a nitride semiconductor laser device obtained from the wafer.
2. Description of Related Art
Nitride semiconductor laser devices, which are utilized in the recording and playing of a Blu-ray disc or the like and other uses, are being actively researched and developed. For example, to record information at a high density, turning laser light on and off fast is required and a laser device is accordingly driven with short pulses of approximately 20 ns. In pulsed operation, where a laser device exhibits a better response when its impedance is smaller, reducing the resistance that is observed during driving is important, as well as reducing the capacitance of the laser device. To reproduce information, on the other hand, a laser device is required to be reliable and raising the electrostatic discharge withstand voltage is important for the reliability of the laser device.
In the manufacture of a nitride semiconductor laser device, the crystallinity of a layered laser structure is improved commonly by digging grooves in a substrate before crystals are grown (see JP 2005-322786 A and JP 2006-190980 A). The grooves make the layers thicker in its vicinity and, in order to efficiently obtain laser devices that have identical optical characteristics from a single substrate, ridges for optical waveguides are formed at a distance from the grooves. This means that, when a plurality of devices, 102 to 104, are set between a groove 101a and a groove 101b as in FIG. 6, which is a top view of a wafer 100, one of the devices (in FIG. 6, the device 104) is always reverse to the rest of the devices (in FIG. 6, the devices 102 and 103) in terms of the placement of the ridge. Specifically, the ridge of the one device (a ridge 104a in FIG. 6) needs to be formed on the left-hand side of the device whereas the ridges of the rest of the devices (ridges 102a and 103a in FIG. 6) are formed on the right-hand side of the devices, or vice versa.
Two types of devices having different structures are thus fabricated from the wafer 100, and need to be discriminated from each other in, for example, a device characteristics test. This is because the two types of devices have different emission spots, and the point of introduction of light into an optical fiber or a tester needs to be changed accordingly when the devices are tested for emission wavelength or the like in a characteristics test. A difference in pad electrode shape, for example, can be used to discriminate one type from the other. In this case, the difference in pad electrode shape needs to be large enough to be recognizable on image in a tester or a chip mounter.
When devices having different structures are to be discriminated from one another by their pad electrode shapes, the difference in pad electrode shape or pad electrode area between the devices needs to be large for easier image recognition, as described above. However, varying the pad electrode area from one structure to another means that devices of different capacitances are fabricated from a single wafer, and creates the following inconvenience.
When a nitride semiconductor laser device is used to record information, a high frequency superposition circuit is usually employed as a countermeasure for optical feedback noise. The high frequency superposition circuit needs to be adjusted for each laser device if laser devices having different capacitances are used. This causes an increase in cost and a reduction in productivity, and is therefore impractical for actual mass production.